Several critical components of commercial and military gas turbine engines are manufactured from titanium-alloy billets. The billets are prepared by melting the titanium alloy of the appropriate composition, casting the titanium alloy as an ingot, and converting the ingot to the billet form. After appropriate mechanical working of the billet to the required thickness and diameter, the component is machined from the billet.
The billet must be readily inspectable by ultrasonic techniques at various stages of the mechanical working process. The ultrasonic inspection detects defects such as cracks, tears, and chemical inhomogeneities that may be present in the workpiece. Such defects, if undetected, are present in the final article and may lead to its premature failure if the defect is sufficiently large. It is absolutely critical that defects of small size be detected during the mechanical working processing, preferably as early in the processing as possible, so that the defect-containing workpieces may be removed from the processing without incurring additional costs or repaired, if that is possible.
Examples of such components include fan disks and compressor disks. These components support respective fan and compressor blades and rotate at high speeds about their shafts during service of the gas turbine engine. If such a disk fails due to the presence of an undetected defect, the gas turbine engine may be torn apart, with catastrophic results for the aircraft.
Alpha-beta titanium alloys are of most interest in fabricating such gas turbine components, because they have desirable mechanical properties that may be tailored by appropriate thermal and thermomechanical treatments. However, the ability to ultrasonically inspect large, thick workpieces of alpha-beta titanium alloys is limited by the attenuation of the ultrasonic inspecting beam due to the microstructural features of the billet. When the attenuation becomes sufficiently great, it is not possible to properly inspect the billet because the strength of the transmitted or reflected ultrasonic signal becomes too small. For this reason, in the critical application requiring good ultrasonic inspectability, the sizes of the billet and of the final article are limited. If it were possible to inspect larger billets ultrasonically, articles could be produced with fewer forging steps, leading to more-economical processing.
There is a need for an improved approach to the conversion of ingots of alpha-beta titanium alloys to billets. The present invention fulfills this need, and further provides related advantages.